In the past, turbine power below 100 hp has been seldomly used in air separation processes because of the prohibitive cost of the generator and associated hardware. Many nitrogen (or oxygen) producing plants use a product compressor to boost the product pressure according to the customer specifications. Some nitrogen plants (with waste expansion), on the other hand, do not have a product compressor and are designed to operate with higher overall pressure, which is determined by the required pressure of the product nitrogen. The waste expansion plants require more energy and, therefore, are less economical to operate. However, this inefficiency has been justified by the capital cost savings associated by eliminating the product compressor. In general, it is desirable for cryogenic air separation plants to have a refrigeration producing expander (turbine) to be self sustainable.
For liquid production using a gas producing plant, it is desirable to increase the size of the turbine and then to modify the process accordingly. Generally, the process engineer cannot increase the size of the turbine independent of other process requirements. Thus, the amount of liquid produced and, many times, especially during hot ambient temperatures days, the capacity of a plant to produce the product gas itself becomes restricted, partly due to refrigeration limitations. Accordingly, it would be beneficial to control the amount of the refrigeration produced according to the requirements of a particular system. Controlling refrigeration by varying the turbine power, without otherwise affecting the process, was not practiced, because such a device was not available.
The effectiveness of small to medium size gas producing plants can be improved if the turbine power is used in other parts of the plant by driving, for example, a booster compressor, which is used to compress either the product stream or even the feed air itself. The problem in the past, aside from capital cost, has been to match the speed and power of both turbine and compressor stages. The booster compressor operating parameters, including pressures (head), flow and power, are mandated by the respective (compressing) stream requirements, either product stream or feed air stream. High efficiency operation usually determines the compressor size and its optimum speed. These booster compressor requirements are independent and decoupled from the turbine operating head, flow and power requirements, which are dictated by other process objectives.
Both turbine and compressor must be allowed to run at their optimum speeds for best efficiency operation. The speeds and powers of the turbine and compressor do not usually match. In the past one would size the turbine first, and then tailor the process to size the booster around the turbine. Usually, one or more process qualities, like flow, speed and/or efficiency is compromised. In the case of a generator loaded capital and operating costs are determinative in developing air separation processes. As a result, turbine power below 100 hp has traditionally been viewed to be uneconomical to recover, and was thrown away, wasted.
As indicated above, in smaller-to-medium size plants, the turbine power was not used, but wasted in a heat rejection loop. If, and when, that power was used to drive a compressor, returning the energy back to the process, it was frequently done with a compromise of the compressor performance, because the powers and speeds of both turbine and compressor would rarely match. Loading the turbine with an electric generator, when economically feasible is still less effective than directly with a compressor.
However, applicant is aware of no art describing the teachings of the present invention, namely, a combination of a cryogenic turbine with an electric motor/generator and a compressor stage (or stages) in one device, with a gear case, to provide optimal operation of both the cryogenic turbine and the compressor. Installing a cryogenic turbine on a warm end machine, such as base load air compressor or gear case, is not practical, because of logistic difficulties with routing and insulating the cryogenic turbine piping at or near the warm end equipment. The latter is usually physically located some distance from the cold box, housing the turbine duct and all cryogenic piping.
The prior art addresses various aspects of expander apparatus design and application. For example, compressor loaded turbine (compander) applications in the cryogenic process industries, and generator loaded cryogenic turbines are known in the art. The use of companders is known in air separation plants for producing low pressure gaseous oxygen and/or nitrogen products. For example, in U.S. Pat. No. 5,268,328, electrical motor is combined with a warm, process air turbine to drive a compressor.
U.S. Pat. No. 4,817,393 discloses that there is little cost difference between driving a small warm end air compressor or an electric generator with that shaft power output. U.S. Pat. No. 4,769,055 discloses that the incremental compression obtained in the warm end compressor is very economical, because the drive power is "free" and the capital cost of the compander is little different from the capital cost of an expander with some other means of absorbing the power developed.